The present invention relates to a printing reflector, and more specifically to a printing reflector for producing consistent shoulders angles on features formed on a flexographic printing plate.
Flexographic printing is a direct or “relief” process, which means that ink is directly transferred from a flexographic plate to a medium, which may be paper, plastic or another suitable material. For this to happen, the print surface “feature” must protrude above the rest of the plate, which means that there must be “relief” around the feature. The low, flat smooth areas of the plate with no features are referred to as the floor. Floor exposures are short and are made through the back of the plate (up side down). Features are dots, lines or solids and all share the same optimum feature profile. Ideal features have a flat printing surface which is supported by a profile which the industry calls the “shoulder.” The optimum shoulder has an angle between 20 and 30 degrees. This optimum shoulder angle provides a support base for features exposed onto the plate but is not so shallow that the features begin to lose relief between adjacent features. Too flat of a shoulder angle results in the area between the features filling in with ink during printing and/or the printed feature size being increased by “squish” which is caused when the plate is pressed against the media, known in the industry as dot gain. Too steep of a shoulder angle results in plate features that “fall over” and don't print. Adjacent features should have a cross section much like a mountain range where the shoulder of one feature comes down to meet the shoulder of the next, but there is no bridging between the features.
The flexographic plate exposure products used to date are based on two designs: (1) bank lighting which is composed of rows of fluorescent tubes positioned approx six inches from the unexposed plate, and (2) a point light source positioned approx 50 inches from the plate. The point light source design is an adaptation of a product designed to serve the lithographic plate-making process.
The advantage of bank lighting systems is even coverage, while the disadvantage is shoulders that rapidly broaden in size until reaching the base of the flexographic plate, due to the physical size of the light array. These shallow shoulder angles of the features on the plate result in a lack of printing sharpness that is undesirable. Another disadvantage is that plate features tend to point toward the nearest fluorescent tube rather than straight up.
The advantage of the point source system is steeper shoulder angles of the features on the plate compared to bank lighting systems, resulting in improved sharpness during printing. The steeper shoulder angle is the result of the effective smaller optical size of the point light source and more parallel rays because of the distance being increased from 6″ to 50″. Unfortunately, the increased distance results in the light rays becoming more parallel than 30 degrees and the resultant shoulder angle (feature support profile) is actually too steep. With this system the optimum feature profile is achieved by positioning a diffuser in the light path at the proper distance from the exposure plane. These diffusers are very inefficient in that they block more light than they transmit, thus producing long duration exposures. A reduction of flexographic plate exposure time is important to printers and is often a key selling point. Additionally, these long exposures caused by the use of the inefficient diffusers generate much more heat than short exposures. This is a known problem inherent with point light source exposure systems. Finally, with a point light source, there is unevenness across the image plane because the light intensity falls off around the outside edges of the plate according to the inverse square law. This is another known problem in the industry. The fact that points further from the center of the plate receive less energy causes those features to be underexposed. This type of system also suffers in quality since the plate features tend to point toward the source.
There is a need in the art for a light source exposure system that addresses the above-described problems with existing flexographic printing systems.